The embodiments described herein relate generally to an ion trap mobility spectrometer (ITMS) and, more particularly, to an ITMS for enhancing detection of materials of interest through enhanced resolution of high-mobility ions and low-mobility ions.
At least some known spectroscopic detection devices include ion trap mobility spectrometer (ITMS) detection systems. Such ITMS detection systems are used to detect trace portions of materials of interest, e.g., residues. At least some known ITMS detection systems include an ionization chamber that produces positive ions, negative ions, and free electrons. As the ions are being generated in the ionization chamber to increase the ion population therein, a retaining grid is maintained at a slightly greater potential than the electric field in the ionization chamber to induce a retention field and reduce the potential for ion leakage from the chamber. An electric field is then induced across the ionization chamber and, depending on the polarity of the induced electric field, the positive ions or the negative ions are pulsed from the ionization chamber, through a high-voltage “kickout pulse”, into a drift region through the retaining grid. The ions are transported through the drift region to a collector electrode. Signals representative of the ion population at the collector electrode are generated and transmitted to an analysis instrument and/or system to determine the constituents in the collected gas samples.
The population of ions generated in the ionization chamber include low-mobility analytes and high-mobility analytes. The low-mobility analytes traverse the drift region with a lower velocity than the high-mobility analytes due to their relatively lower mass than the lighter high-mobility analytes. The low-mobility and high-mobility analytes pulsed into the drift region from the ionization chamber typically form an ion disk with a predetermined axial width value and possibly a trailing ion tail. Such trailing ion tail defines an asymmetric peak trace on spectral analysis equipment that negatively impacts the subsequent analysis of the peak trace. The ideal peak trace for spectral analysis is perfectly symmetrical.
Further, in many known ITMS detection systems, as the disk of ions traverses the drift region, the separation of the high-mobility analytes from the low-mobility analytes induces expansion and distortion of the ion disk. The high-mobility analytes form a disk that transits faster than a disk formed of low-mobility analytes and the disks may overlap as they are received at the collector electrode. The peaks on the trace thus generated on the spectral analysis equipment is distorted with poor resolution and are difficult to analyze. Moreover, in many of the known ITMS detection systems, there is no precise control over the width of the ion disk injected into the drift region. Fundamentally, this is due to inconsistent, and sometimes, incomplete clearing out of the ionization chamber due to nonhomogeneity of the electric field induced in the ionization chamber, e.g., low field regions at the back of the ionization chamber.
Increasing the strength of the electric field to empty the ionization chamber more rapidly and to decrease the transit time through the drift region increases the potential for ion leakage from the ionization chamber through the retainer grid after the kickout pulse. Such ion loss decreases the resolution of the spectral peaks to be analyzed. Increasing the width of the kickout pulse to eject a greater number of slow ions of interest without losing a significant portion to the retention grid may increase the width of the detected peaks of the reactant ions and analyte peaks of interest. Such an increase in peak width decreases the resolution of the analyses in the region typically associated with HME substances.
Further, increasing the field strength for a kickout pulse of reduced width to eject both high-mobility ions and low-mobility ions may result in the ions just inside the chamber proximate the retention grid to induce an electric field of their own that opposes the retention field generated by the retention grid. Moreover, if the kickout pulse is reduced in width, a significant ion tail develops on the ion disk. The peak trace also develops an asymmetric peak trace on the spectral analysis equipment due to the detection peaks associated with ions continuing to leak through the retention grid following cessation of the pulse as the ions just inside the grid create a field of their own in opposition to the retention voltage field. As such, the resolution of the instrument/system is reduced.